Update: I was able to schedule a system reboot, so I removed the kernel command line option preempt=full and restarted the system Guess what, that completey solved the problems! While still using gdm, the cursor no longer flickers anywhere. And there is no more transient garbage square under the cursor. I tested with the mouse cursor oing over various icons, menus, buttons, on web browser content (both Firefox an Chrome).
I tried changing the displaymanager from gdm to lightDM, and it made no difference. Should I even see anything different with lightdm rather than gdm? Anyway, there were no problems with the mouse cursor in this setting either.
I then switched back to gdm, and re-enabled compositing, then I logged out and back again. No problem with the mouse cursor!
At this point I guess I consider the problem “solved”, even though I don’t know why removing the preempt=full kernel command line made a difference. It seems Leap 15.4 adds this option by default, it’s on all four of the Leap 15.4 systems I have here. Yet preemmpt=full does not cause any apparent problems on all the other systems. Just this particular one.
Oh, and I “kind of” figured out why the X display resolution was listed as 5120x1440 but the physical screen resolution is 2560x1440. It seems that the X server allocated a much wider display than the screen itself. There is actually more display area beyond the right side edge. You can see this in the vncviewer, where tht right side is just a block area beyond 2560 pixels. It turns out that something caused the system to run nvidia-xconfig (maybe during an update), and generated a new /etc/X11/xorg.conf file. If I restore the old xorg.conf and restart the X session, then I get the following:
dimensions: 2560x1440 pixels (677x381 millimeters)
resolution: 96x96 dots per inch
depths (7): 24, 1, 4, 8, 15, 16, 32
inxi -GSaz output:
Kernel: 5.14.21-150400.24.18-default x86_64 bits: 64 compiler: gcc
splash=silent quiet mitigations=auto security=apparmor
Desktop: KDE Plasma 5.24.4 tk: Qt 5.15.2 wm: kwin_x11 vt: 7
dm: GDM 41.3, SDDM Distro: openSUSE Leap 15.4
Device-1: NVIDIA GK208B [GeForce GT 730] vendor: eVga.com. driver: nvidia
v: 470.141.03 alternate: nouveau,nvidia_drm bus-ID: 01:00.0
chip-ID: 10de:1287 class-ID: 0300
Device-2: Logitech Webcam C600 type: USB driver: snd-usb-audio,uvcvideo
bus-ID: 1-3:4 chip-ID: 046d:0808 class-ID: 0102 serial: <filter>
Display: x11 server: X.Org 1.20.3 compositor: kwin_x11 driver:
loaded: modesetting unloaded: fbdev,vesa,vmware alternate: vboxvideo
display-ID: :0 screens: 1
Screen-1: 0 s-res: 2560x1440 s-dpi: 96 s-size: 677x381mm (26.7x15.0")
s-diag: 777mm (30.6")
Monitor-1: DVI-D-1 res: 2560x1440 hz: 60 dpi: 118
size: 552x311mm (21.7x12.2") diag: 634mm (24.9")
OpenGL: renderer: llvmpipe (LLVM 11.0.1 256 bits) v: 4.5 Mesa 21.2.4
compat-v: 3.1 direct render: Yes
Note that the line “Screen-1: 0 s-res: 2560x1440” rather than “Screen-1: 0 s-res: 5120x1440” shown before.
Any comments about these findings?